The inventors of the present invention have studied the following technologies regarding a semiconductor integrated circuit device such as a system LSI or a microprocessor for mobile devices.
In recent mobile devices, I/O (input/output) voltages have been more and more diversified in order to achieve the reduction in voltage for lower power consumption and also to use the conventional interface that operates at a high voltage for effectively utilizing the existing resources. With respect to the voltage reduction, in particular, demands for the lower power consumption have been strong in an interface unit of a dynamic random access memory (DRAM), which is a general-purpose storage element. Such demands serve as an engine of standardization of low-voltage I/O. For example, in mobile devices, the voltage of 1.8 V which is lower in comparison with the current industry standard of 3.3 V has been becoming the standard interface voltage for SDRAM, DDR-SDRAM, and others.
On the other hand, it is also important to keep the conventional interface. This is because a removable non-volatile memory (such as flash memory) is designed to operate at a high voltage (3.3 V), and its interface specifications are also standardized in the industry. Such a conventional interface has been adopted to various types of devices of different product generations, and has an advantage of cost reduction by a mass-production effect. Therefore, demands for the continued use of such a conventional I/O are also high.
Therefore, at present, in consideration of cost and power reduction, it is extremely difficult to unify all power supply voltages for I/O circuitries (input/output circuitry) incorporated in an LSI (for example, to unify all power supply voltages into a voltage of 1.8 V).
Meanwhile, for low-voltage (1.8 V) I/O, a high-speed operation has not been much demanded so far. Thus, for 1.8 V I/O, a transistor supposed to be operated at a standard voltage (for example, 3.3 V) can be used as being operated at a low voltage (1.8 V). However, recent mobile devices are better equipped with a variety of applications, and needs for transferring a large amount of data at high speed have been increasing. Therefore, demands for a high-speed operation have been increasing even in I/O of a low-voltage operation. In the future, speeding up of 1.8 V I/O will be imperative.